Existing lasers vary greatly with respect to many aspects including, for example, the crystalline host, the dopant materials for the host, the operating power, the operating wavelength, the cavity design, the method of pumping and the mode discipline used (mode-locking, single frequency, or chaotic operation). The single most frequent way in which lasers are identified is by the type of crystaline host or gain material utilized within the laser, since this material will strongly influence, if not dictate, the other considerations for laser design.
The efficient room temperature operation of Tm-doped yttrium-aluminum-garnet (YAG) as a solid state laser material has led to the emergence of such materials as an important source of tunable, coherent radiation in the 2 .mu.m wavelength region. Continuous wave (diode-pumped), pulsed (flashlamp-pumped) and Q-switched operation of these solid state lasers have generated considerable interest in the use of these lasers in applications such as lidar and medicine. Additionally, ultra-fast pulses generated from these lasers are also usable in spectroscopy, optical communications and nonlinear wavelength conversion applications.
To be useful in some of the above applications, the laser must be able to generate optical pulses having a duration in the picosecond range as well as a repetition rate in the MHz range. One method to accomplish this is to utilize an acousto-optic (AO) modelocker disposed in a lasing cavity to control the pulse duration and repetition of the pulses generated by the laser. Active modelocking techniques have been successfully applied to solid-state lasers generating pulses of picosecond (ps) duration at high repetition rates.
Active modelocking techniques utilizing some form of amplitude modulation (AM), phase or frequency modulation (FM), or a combination of both have been successfully applied to several solid state laser systems. Modelocking through amplitude modulation, i.e., synchronous pumping, relies on nonlinear gain dynamics for pulse formation and is typically reserved for gain media having large stimulated-emission cross sections (.gtoreq.10.sup.-17 cm.sup.2). In contrast, for rare-earth media, such as thulium, that have relatively smaller gain cross section, active modelocking by direct modulation of the loss is more applicable. Modelocking is obtained by modulating the cavity loss at a rate equal to the inverse of the resonator round trip time.
Examples of devices which utilize the active modelocking techniques described above includes a number of modelocked Nd:based lasers which can generate pulses as short as 7 ps at pulse repetition rates ranging from tens of MHz to a few GHz. U.S. Pat. No. 4,951,294 (Basu et al.) discloses the use of an acousto-optic mode locker in an optical cavity to generate picosecond pulses from a Nd:YAG laser which is optically pumped by a laser diode. U.S. Pat. No. 4,764,933 (Kozlovsky et al.) discloses a diode pumped Nd:Glass laser which utilizes an acousto-optic modelocker for generating an output signal comprising short pulses. In another example, modelocked operation of a flash-pumped 2.1 .mu.m Ho:YAG laser has been provided which generates pulses of 590 ps duration.
Another problem associated with lasers of this type is the need to accurately control the length of the laser cavity. Modelocking is obtained by modulating the cavity loss at a rate equal to the inverse of the cavity round trip time. The round trip time may be controlled by adjusting the length of the laser cavity. Therefore, these parameters are of crucial importance in determining pulse width. In addition, noise produced due to pump laser plasma instabilities, dye jet fluctuations and thermal expansion poses severe difficulties in laser operation.
Several laser devices have been constructed to specifically address this problem. For example, U.S. Pat. No. 4,461,006 (Salour et al.) discloses a laser device in which a cadmium sulfide laser crystal is pumped by an Ar laser. The laser cavity is defined by two mirrors, the first of which is permanently fixed at a distance L.sub.1 from the laser crystal and the second mirror which is disposed at a distance L.sub.2 which is adjustable to hundredths of a micron. By appropriate adjustment of the second mirror so as to substantially match the length of the laser cavity to the cavity length of the pumping laser, output pulses as short as 8 ps can be obtained.
In applications such as nonlinear wavelength conversion, modelocked lasers are of considerable use, specifically as a pump source for mid infrared (IR) parametric oscillators and amplifiers. Pump sources for such applications must, however, be capable of delivering high peak power outputs. One technique for attaining the required peak powers is by injection modelocking of a Q-switched laser oscillator. For this technique, ultra-short pulses from a continuous wave (CW) modelocked laser are used to seed a Q-switched laser oscillator, thereby producing a packet of high peak power pulses under the giant pulse envelope. For example, U.S. Pat. No. 4,965,803 (Esterowitz et al.) discloses a room temperature, diode pumped, thulium-doped solid state laser which utilizes a Q-switch. Another example of a laser utilizing a Q-switch is that disclosed in U.S. Pat. No. 5,099,486 (Acharekar et al.). This patent discloses a frequency stabilized Ho:YAG laser which is pumped by a flashlamp. The output of the Ho:YAG laser is reflected in a ring path. Disposed in this ring path is a Q-switch, and a mirror and associated piezoelectric transducer for adjusting the mirror. A seed laser is provided to set a reference frequency for the Ho:YAG laser output. In operation, the time period between the initial lasing output of the Ho:YAG laser and the activation of the Q-switch is measured. This time information is utilized to adjust the mirror in the optical path so as to cause the Ho:YAG laser to lase at a frequency which is defined by the seed laser.